A LED projector has been proposed that has a light source device that employs a light-emitting diode (LED) as a light source. Compared to a discharge lamp, an LED, which is a solid-state light source, has the advantages of being mercury-free, capable of quick-start/quick-off lighting operation, and highly durable. An LED projector of this type of the prior art is made up of a light source device, illumination optics into which light from the light source device is irradiated, a light valve having a liquid crystal display panel into which light from the illumination optics is irradiated, and projection optics for projecting light from the light valve onto a projection surface.
In order to increase the luminance of a projected image in this type of LED projector, it is desirable that all possible measures be taken to limit optical loss in the optical path from the light source device to the projection surface.
The two chief causes of optical loss that can be considered are as described below.
The first cause is polarization loss that arises from polarization dependence of a liquid crystal display panel or dichroic prism.
The second cause is the occurrence of light that diverges and does not enter each optical element arranged on the optical path, i.e., optical loss resulting from eclipse in each optical element. This occurs because light from a light source device cannot be used as projection light unless the product of the angle of radiation (solid angle) of emitted light from optical elements belonging to the light source device and the light emission area (etendue) is made lower than the value of the product of the area of the display element and the acceptance angle (solid angle) that is determined by the f-number of the projection lens.
As one countermeasure for solving the above-described problems relating to optical loss, the use of an optical element made up of a hologram and a light guide body has been proposed with the object of generating light that is irradiated into a liquid crystal display element.
As one example of an optical element that uses a hologram and a light guide body, Patent Document 1 discloses optical element 101 that is provided with: light guide body 103 into which light from light source LED 102 is irradiated; hologram 104 that diffracts light from this light guide body 103; polarization-conversion layer 105 that imparts a predetermined phase difference to mutually orthogonal polarization components of light that is transmitted through; and polarization-separation layer 106 that emits only linearly polarized light having a polarization component of a specific direction from this polarization-conversion layer 105, as shown in FIG. 1.
As another example of an optical element that uses a hologram and a light guide body, Patent Document 2 discloses optical element 111 that is provided with: light guide body 113 into which light from cold-cathode tube 112 as the light source is irradiated; hologram 114 that diffracts only linearly polarized light having a polarization component of a specific direction from this light guide body 113; quarter-wave plate 115 into which light that was diffracted by this hologram 114 is irradiated; and reflection plate 116 that reflects light that has been transmitted through this quarter-wave plate 115, as shown in FIG. 2. In addition, in this optical element 111, diffuser plate 117 into which polarized light that has been diffracted by hologram 114 is irradiated, polarization plate 118, liquid crystal cell 119, and polarization plate 120 are each arranged along the optical path. In this optical element 111, reflection plate 121 that reflects light from cold-cathode tube 112 is provided, and light from cold-cathode tube 112 is converted to parallel light by reflection plate 121 and irradiated into light guide body 113.